Many structures (including stationary structures, such as bridges, buildings, or the like, as well as moving structures or machines, such as vehicles, cranes, wind turbines, or the like) operate in environments with unpredictable conditions, exposing the structures to high levels of mechanical loading, including extreme loading and cyclic fatigue loading. To increase reliability and operating lifetime, such structures are often made with costly high strength materials and are engineered to avoid failure under high levels of loading. However, high levels of loading may occur sporadically in some environments and therefore might be experienced relatively infrequently over the lifetime of a structure. Accordingly, such structures are often over-engineered and/or operated in an overly conservative mode for the conditions they are exposed to most of the time, thereby reducing overall return on investment.
Moreover, predicting the levels of loading to which a structure might be exposed over its lifetime can be difficult, particularly when some of the sources of loading are non-deterministic, such as weather conditions, terrain conditions, and/or operator actions. For example, aircraft are subject to non-deterministic weather conditions and operator actions. As another example, a ground transport structure, such as a fuel tanker, is subject to non-deterministic terrain conditions and operator actions. A third example, which is referred to throughout the following description by way of example but not limitation, is a wind turbine generator (WTG). WTGs, in particular the blades of WTGs, are subject to cyclic fatigue loading as well as extreme loading due to non-deterministic wind conditions over their operating lifetime.
A WTG is an energy conversion system that converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind incident on blades of the WTG causes a rotor of the WTG to rotate. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. Because wind conditions can vary significantly from one location to another, a WTG is typically engineered or selected from among different reliability classes of WTGs to withstand the expected wind conditions of the location.
However, as noted above, because weather conditions are non-deterministic, the expected wind conditions as measured when siting a location might not fully represent the actual wind conditions experienced by the WTG after installation. Consequently, the loading experienced by the WTG (or portions thereof) can exceed levels for which the WTG is designed. Discrepancy between predicted wind conditions and actual wind conditions could result from a number of sources including, for example, normal variations in wind conditions from year to year, or insufficient information in wind condition measurements. Many WTGs have the ability to detect extreme wind conditions, including wind gusts and high levels of wind shear, and are often derated or shut off to avoid excessive damage upon detection of extreme wind conditions. However, accurate detection of extreme wind conditions is difficult and, even if wind conditions are measured accurately, they are a crude proxy for the actual loading experienced by the WTG due to the wind conditions. These and related limitations of the present state of the art significantly constrain the efficient operation of certain structures, such as WTGs, and impose the use of unnecessarily conservative control measures to avoid the risk of damage and extend operable lifetime.
At least one system has been developed for sensing and controlling loads on a WTG. For example, U.S. Pat. No. 7,160,083 (“the '083 patent”), issued to Pierce et al. on Jan. 9, 2007, describes a method and apparatus for reducing fatigue loading on turbine components. Particularly, the system of the '083 patent receives signals from load sensors, determines a load condition based on the signals, and determines a response to the determined load condition. For example, the pitch of the turbine's blades may be altered to reduce loading. While the system of the '083 patent may be effective for sensing the occurrence of and reducing loads on a wind turbine rotor, the system does not appear to address cyclic fatigue loading, which may be quite small in amplitude but results in damage over time. In addition, some forms of loading on a structure component may be expected or desired. However, the system of the '083 patent does not appear to distinguish desired loading from undesired loading and therefore appears to treat all forms of loading as undesirable.